Rattling noise in engine

Hello, I'm so glad Bob & Ken you're there! I have a 1988 Dodge Grand Caravan 6 cylinder with a rebuilt 1992 engine installed in it 2 years ago. Here a little history of the the issues that I'm been having. The last month the check engine light has been coming on first thing in the morning when it is cold. It takes me having to turn the ignition on and letting it crank up 3 / 4 times and then turning it off and waiting to turn the ignition on again until the voltage gauge is in the middle of the battery emblem and then turning it over to start. It normally starts by the second or third time. If I don't let it warm up the check engine light frequently comes on after I start to drive and stays on until I get to where I'm going, but then it doesn't always come back on when I get back into the car. Sometimes in the morning the check engine light comes on right away. So I then turn the engine off and on until the check engine light doesn't come on again. Today I noticed a rattle inside of the engine, got louder on the way home. When it was going up a steep grade it then started overheating, so I pulled off the freeway and it died. I was able to start it up again right away, but waited until it cooled off. There was plenty of water & coolant in the radiator. Oil was changed 3 weeks ago & it was also fine. After cooling down I started it up again & continued home for another 15 miles whereby it got warm but didn't overheat. The rattling noise in the engine continued. Question : Is the rattling noise possibly a loose or broken spring atop the valve? Is the check engine light a faulty sensor? How is the overheating related to all of this or is it a separate issue? Desparatlely needing your help. Thank You Arlene